The effects of gonadotrophin releasing hormone administration four days after insemination on first-service conception rates and corpus luteum function in dairy cows.

One hundred and eighty-five Holstein-Friesian dairy cows received either sterile water or 250 micrograms of gonadotrophin releasing hormone intramuscularly on the fourth day after the first service postpartum. Heparinized blood samples were taken immediately prior to treatment (day 4) and on day 8 postinsemination for analysis of plasma progesterone concentration. Pregnancy diagnosis was carried out by rectal palpation at 42 days postinsemination and reconfirmed after 60 days postbreeding. The pregnancy rates after first, second or third service were not significantly different between gonadotrophin releasing hormone-treated and control cows. Plasma progesterone concentrations on day 4 and day 8 postinsemination, as well as the change in plasma progesterone concentration from day 4 to day 8, were similar for gonadotrophin releasing hormone-treated and control cows. The plasma progesterone concentrations on day 8 postbreeding were significantly higher (p less than 0.005) and the change in progesterone concentrations between days 4 and 8 were significantly greater (p less than 0.002) in pregnant cows compared to nonpregnant cows.